Protein kinase C is an enzyme found in nearly all animal tissues and animal cells that have been examined. Its identity is generally established by its ability to phosphorylate proteins when adenosine triphosphate, calcium ions and phospholipid cofactors are present, with greatly reduced activity when these cofactors are absent. Protein kinase C activity is substantially stimulated by certain 1,2-sn-diacylglycerols that bind specifically and stoichiometrically to a recognition site on the enzyme. Stimulation of protein kinase C by these diacylglycerols has been shown to be an important physiological event that mediates the actions of a wide variety of hormones, neurotransmitters, and other biological control factors such as histamine, vasopressin, alpha-adrenergic agonists, dopamine agonists, muscarinic cholinergic agonists, platelet activating factor, etc. see Y. Nishizuka, Nature 308 693-698 (1984) and Science 225 1365-1370 (1984) for reviews!.
The biological role of protein kinase C is also of great interest because of the discovery that certain very powerful tumor promoting substances activate this enzyme by binding specifically and with very high affinity to the diacylglycerol binding site on the enzyme. In addition to diacylglycerols, there are at present five other known classes of compounds that bind to this site, including diterpenes such as the phorbol esters, indole alkaloids (indolactams) such as the teleocidins, lyngbyatoxin, and indolactam V, polyacetates such as the aplysiatoxins and oscillatoxins, certain derivatives of diaminobenzyl alcohol, and the macrocyclic lactones of the bryostatin class. The phorbol esters have long been known as powerful tumor promoters, the teleocidins, aplysiatoxins and diacylglycerols are now known to have this activity, and it appears likely that additional classes of compounds will be found to have the toxic and tumor promoting activities associated with the capability to bind to the diacylglycerol site of protein kinase C and thus activate the enzyme. Other toxicities of these agents when administered to animals include lung injury and profound changes in blood elements, such as leukopenia and neuropenia.
In addition to potent tumor promoting activity, these six classes of compounds, collectively referred to as the "phorboids", display a vast range of biological activities, as would be expected from the widespread distribution of their target enzyme. Some of these activities, like tumor promotion, indicate the involvement of protein kinase C in important normal or pathological processes in animals. Thus, the phorboids are potent skin inflammatory agents, cause smooth muscle contraction in several tissues, alter immune system function and can be used to cause a variety of other normal or pathological responses. Related disease states such as the development of cancer, the onset and/or maintenance of inflammatory disease, the role of vasoconstriction in hypertension, the role of bronchoconstriction in asthma, the role of cholinergic, adrenergic, and dopaminergic synapses in diseases of the central/peripheral nervous systems, may be mediated in vivo by the stimulation of protein kinase C by diacylglycerols, the latter being generated in the cell by pathological agents or conditions.
In analyzing the activity of a pharmaceutical or other bioactive compound, it is useful to consider two properties: the efficacy, defined as the capability to elicit a full or partial biological result, such as complete displacement of a ligand from its receptor site or the complete inhibition of inflammation or edema caused by a standard stimulus; and the potency, defined as that amount or concentration of drug that causes 50% of the full response (often abbreviated as the ED.sub.50). It is frequently the case within a given class of pharmaceutical agents that individual members of the class all have equal efficacy, i.e. they each can generate a full biological effect, but they show differing potencies. Thus, the structural modifications within such a class affect only the amount necessary to achieve a given result, and the modified compounds otherwise have generally the same central biological characteristic. There may also be differences between members of such a class as regards properties other than the central biological characteristic; for example, members of the class might differ in side effects or toxicity.
Well-known pharmaceuticals that have been in extensive use for years or decades show a wide range of optimal therapueutic potencies. Aspirin, for example, is often taken in multi-gram amounts per day for treatment of inflammation or arthritis, and detailed analyses of its mechanism of action in vitro show that a concentration in the millimolar range is required. In contrast, steroid-based topical anti-inflammatory compounds such as fluocinolone acetonide are many thousand-fold more potent, and, beyond this, some oral contraceptive agents are prescribed in daily doses in the micro-gram range. Thus, although high potency is generally advantageous for a pharmaceutical, it is not an absolute requirement.
Several thousand of the high skin-inflammatory and tumor-promoting phorboids have been reported in the literature, including numerous examples on which minor chemical modifications have been made see Evans and Soper, Lloydia 41 193-233 (1978) and references cited therein!. The structures of these phorboids can be compared, and their activities for inflammation and tumor promotion can be analyzed from the perspective of efficacy and potency. The structures of the different classes of phorboids vary quite markedly from one to the other class (see Summary of the Invention for structural comparisons), yet widespread testing of their biological activities has shown that these classes have generally very similar biological properties. In particular, the thousands of known phorboids of the highly potent diterpene, indolactam, and polyacetate classes appear to have, with very minor exceptions, virtually identical efficacies as skin irritants and tumor promoters T. Sugimura, Gann 73 499-507 (1982)!. The exceptions involve a few compounds that have a short duration of irritant activity and/or manifest diminished tumor promoting activity, perhaps due to toxicity or secondary parameters such as differing metabolic destruction rates.
In contrast to the essentially equal efficacies among the vast majority of phorboids, their relative potencies cover a wide range, as measured in inflammation and promotion tests and as measured in numerous other in vivo and in vitro systems. Example compounds can be found in the diterpene, indolactam, and polyacetate classes that have nearly equal, very high potencies. At the same time there are compounds in each of these classes which embody significant structural changes that do not diminish efficacy but do result in potency decreases of 10-fold to 100,000-fold or more see, for example, Driedger and Blumberg, Cancer Res. 37 3257-3265 (1977), Cancer Res. 39 714-719 (1979)!. Thus, all these compounds appear to be capable of achieving generally the same biological results, and merely differ in the amount which must be used to obtain a given result.
In vitro measurements of biochemical properties provide an even more sensitive method for comparing the properties of the various phorboids. For example, using a radioactively labeled phorboid such as .sup.3 H!phorbol 12,13-dibutyrate or .sup.3 H!lyngbyatoxin, one can measure the potency of a test compound as a competitive ligand for the diacylglycerol binding site on protein kinase C. Alternatively, one can measure the ability of a given phorboid to stimulate the protein kinase C-mediated incorporation of radioactive phosphate from .sup.32 P!adenosine triphosphate into a standard acceptor substrate such as histone H1. Tests of this nature reveal a difference in potency between given phorboid agonists of as much as 10,000,000-fold or more Dunn and Blumberg, Cancer Res. 43 4632-4637 (1983), Table 1!.
These basic data regarding the phorboid agonists are an important consideration because they underscore the concept that the structural differences among these previously known phorboids, especially the diterpenes, indolactams, polyacetates, and bryostatins, generally do not affect their efficacies as toxic agonists, and indeed a wide variety of structural changes are tolerated in this regard. Such changes generally alter potency only and do not provide agents with therapeutic utility, since they retain their toxicity.
Some minor changes in phorboid structure are known to result in inactive compounds, such as a stereochemical change from 4-beta to 4-alpha in the phorbol series, and indeed some of the diterpene skeleton structures carry hydroxy groups that must be esterified in order for inflammatory activity to be observed. However, these inactive compounds are quite few in number among the known phorboids, and no therapeutic utility has been demonstrated for them.
The phorbol esters, indolactams, polyacetates, diaminobenzyl alcohols, and bryostatins are generally found in plants, molds, and algae, or are synthetic in origin. Although they are found in many parts of the world, normal human contact with them is thought to be low. In contrast, the diacylglycerols are part of the functioning of virtually every type of animal cell except erythrocytes of some species, and thus the undesirable activation of protein kinase C by the diacylglycerols may have a very widespread role in human diseases.
Thus, new compounds, capable of blocking the activation of protein kinase C by acting as specific pharmacological antagonists of the diacylglycerols, would be valuable agents in the prevention and treatment of a wide variety of diseases in animals and humans.
There may be several different forms of protein kinase C each having different biological roles. The stimulation of one form can lead to undesirable results such as inflammation or the development of cancer, while stimulation of another form of the enzyme might produce beneficial effects, such as the abrogation of inflammation or the secretion of useful bioregulatory factors such as hormones and interferons.
Moreover, the exact correspondence between diacylglycerol binding sites and protein kinase C has not been fully explored, and there may be several different such binding sites with differential affinities for diacylglycerols and other phorboids. Indeed there is published evidence for several distinct classes of phorboid binding sites in various tissues and cell types (Dunn and Blumberg, op. cit.). However, in this study, even the ligands showing the clearest differences in affinity for these distinct classes are only selective by a factor of 10-100 in dissociation constant. Thus, other new compounds, capable of selectively activating one useful, but not another, deleterious, diacylglycerol target site, would also be valuable agents for the prevention or treatment of inflammation.
Earlier efforts to use the previously known phorboids themselves or to modify the structures of the known phorboids, have not been successful in producing useful compounds.
It has been known for some time that several of the toxic, inflammatory and tumor-promoting compounds such as phorbol 12-tigliate 13-decanoate, mezerein, lyngbyatoxin and aplysiatoxin have anti-leukemic activity in mouse model tests. However, these compounds are all extremely toxic and are cancer suspect agents, thus eliminating them from consideration as human therapeutic agents.
Ganong et al. Proc. Nat. Acad. Sci. 83 1184-1188 (1986)! tested a series of diacylglycerols and found no antagonistic activity in that series against the standard agonist 1,2-dioctanoylglycerol. It is of particular note that several compounds tested in this work were modified in the hydroxymethyl portion of the diacylglycerol molecule, and these modifications produced only a loss of activity or a weakened activity that was not distinguishable from the agonist activity of 1,2-dioctanoylglycerol itself. These hydroxymethyl-modified compounds were not antagonists in these tests and no utility was found. Similarly, Thielmann and Hecker Forsch. Krebsforsch. Vol. VII, pp. 171-179 (1969), New York: Schattauer! found only a complete loss of biological activity in their study when the hydroxy group of the hydroxymethyl on phorbol 12,13-didecanoate was replaced with hydrogen or chlorine. Schmidt and Hecker H. Lettre and G. Wagner (eds.), Aktuelle Probleme aus dem Gebiet der Cancerologie, Vol. III, 3rd Heidelberg Symposium, pp. 98-108. Berlin: Springer Verlag, 1971.! also found that oxidation of the hydroxymethyl of phorbol 12,13-didecanoate to a carboxylic acid caused complete loss of activity in the assays used.
The hydroxymethyl group of the known phorboids (discussed in more detail in Summary of the Invention) has been thought to be required for biological activity, as detailed by Hecker (Carcinogenesis. Vol. 2, eds. Slaga, Sivak and Boutwell, Raven Press, New York, 1978, pp. 11-48 and references cited therein). Indeed, it is stated therein that the replacement of the 20-hydroxyl in a phorbol ester "results in complete loss of biological activity". In another study, replacement of the hydroxy group of the hydroxymethyl (located at carbon 14) by chlorine or hydrogen in indolactam V gave rise to compounds with agonist activity weaker than but otherwise not distinguished from the agonist activity of the very toxic teleocidin class of tumor promoters Irie et al., Int. J. Cancer 36 485-488 (1985)!. Thus no utility was found.
Schmidt and Hecker (Carcinogenesis, Vol. 7, ed. by E. Hecker et al., Raven Press, New York, 1982, pp. 57-63) studied the abilities of a series of diterpene phorboids to inhibit tumor promotion by the standard phorboid agonist tumor promoter phorbol 12-myristate 13-acetate (PMA). They found that, at low doses, some short-chain ester derivatives of phorbol were able to block the tumor promotion by PMA. However, all of the compounds that were active as antagonists at low doses are also very efficacious skin irritants themselves at higher doses and most of them are also known to have tumor promoting activity. Thus, these short-chain esters still have toxic inflammatory and tumor promoting activity and thus have no therapeutic value. In this publication it was also noted that a phorbol 12-ester, namely phorbol 12-myristate, was without activity as an antagonist of PMA-induced tumor promotion. Thielmann and Hecker (op. cit.) also found phorbol 12-decanoate and phorbol 12-myristate to be inactive in the functional tests used, and stated that the 13-OH must be esterified to obtain active compounds.